1. Field of the Invention
The present invention generally relates to a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM). More particularly, the present invention relates to an apparatus and method for correcting an initial carrier frequency offset in a wireless communication system based on OFDM.
2. Description of the Related Art
Wireless communication systems typically make use of a cellular communication scheme. These wireless communication systems make use of multiple access schemes for simultaneous communication with multiple users. For the multiple access schemes, Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA) are typically used. With the rapid progress of CDMA technology, CDMA systems are developing from a voice communication system into a system capable of transmitting packet data at high speeds.
In order to overcome limitations in using code resources of the CDMA system, an Orthogonal Frequency Division Multiple Access (OFDMA) scheme has been used recently.
The OFDMA scheme is based on Orthogonal Frequency Division Multiplexing (OFDM). An OFDM system for transmitting data using multi-carrier is a type of Multi Carrier Modulation (MCM) system in which a serial symbol stream is converted into parallel symbol streams and is modulated into multiple subcarriers, that is, multiple subcarrier channels, orthogonal to each other.
The MCM-based OFDM scheme was first applied to High Frequency (HF) radio communications for the military in the late 1950's. The OFDM scheme for overlapping orthogonal subcarriers started to be developed in the 1970's. Since a problem exists in that it is difficult to implement orthogonal modulation between multiple carriers, the OFDM scheme has limitations in actual system implementation. However, in 1971, Weinstein, et al. proposed that OFDM modulation/demodulation can be efficiently performed using Discrete Fourier Transform (DFT). Thus, the OFDM technology has rapidly developed. Also, the introduction of a guard interval into which a Cyclic Prefix (CP) symbol is inserted further mitigates adverse effects of multipath propagation and delay spread on an OFDM system.
As a result, with the development of technology, the OFDM scheme has been widely used for digital transmission technologies such as Digital Audio Broadcasting (DAB), digital television (TV), Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode, (WATM), and the like. Although hardware complexity is an obstacle to implementation of the OFDM system, recent advances in digital signal processing technology including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) enable the OFDM system to be implemented. The OFDM scheme is analogous to a conventional Frequency Division Multiplexing (FDM) scheme, and can obtain optimal transmission efficiency when high-speed data is transmitted while maintaining orthogonality between multiple subcarriers. More specifically, the OFDM scheme leads to efficient frequency use and is robust to multipath fading, thereby obtaining optimum transmission efficiency upon transmission of high-speed data. The OFDM scheme uses overlapping frequency spectra, thereby efficiently using frequencies. The OFDM scheme is robust to frequency selective fading, multipath fading, and impulse nose. The OFDM scheme can reduce Inter Symbol Interference (ISI) using guard intervals and can easily design an equalizer structure in hardware. Therefore, the OFDM scheme is actively exploited in communication system structures.
FIG. 1 is a block diagram illustrating physical layers for transmission and reception in a conventional OFDM system.
An input bit stream 101 to be transmitted is input to an encoder 102. The encoder 102 encodes the input bit stream 101 in a predefined scheme and then outputs the encoded bit stream to a Serial-to-Parallel Converter (SPC) 103. The SPC 103 converts the encoded serial bit stream into parallel bit streams and then outputs the parallel bit streams for which an IFFT process is performed. Thus, the parallel bit streams output from the SPC 103 are input to an IFFT processor 104. In this case, it is assumed that the parallel bit streams are N symbols. Further, it is assumed that the IFFT processor 104 receives the N symbols because the IFFT process is performed in a unit of N bit streams. Thus, the IFFT processor 104 receives the N parallel symbols and performs the IFFT process for the N parallel symbols to be transmitted, thereby transforming frequency domain symbols into time domain symbols. The time domain symbols are input to a Parallel-to-Serial Converter (PSC) 105. The PSC 105 converts the N time domain symbols input in parallel into N serial or sequential bit streams and then serially or sequentially outputs the N bit streams. Hereinafter, the sequentially output N bit streams are referred to as “OFDM symbols”.
An OFDM symbol is input to a CP adder 106. The CP adder 106 copies a predefined number of last bits of the input OFDM symbol and then inserts the copied bits before a first bit of the OFDM symbol. A CP is added to remove the effect of a multipath channel. The OFDM symbol to which the CP has been added is input to a Digital-to-Analog Converter (DAC) 107. Then, the DAC 107 converts input digital symbols into analog symbols and transmits the analog symbols to a receiver.
The transmitted analog symbols are input to the receiver through a predefined multipath channel 110. Now, a structure and operation of the receiver will be described.
An Analog-to-Digital Converter (ADC) 121 of the receiver receives analog signals transformed into the time domain in the IFFT processor 104 of a transmitter and then converts the received analog signals into digital signals. The ADC 121 inputs the digital signals to a CP remover 122. The CP remover 122 removes CPs, that is, CP symbols, contaminated in a multipath environment. A signal from which the CPs have been removed in the CP remover 122 is a serial signal. Thus, the signal from which the CPs have been removed is input to a SPC 123. The SPC 123 converts serially input symbols into parallel symbols in a unit of N symbols and then outputs the parallel symbols.
The serially input symbols are converted into the parallel symbols in the unit of N symbols because the transmitter performs the IFFT process in the unit of N symbols. Thus, an FFT processor 124 receives N parallel data and then performs an FFT process for the received parallel data. That is, the FFT processor 124 transforms time domain symbols into frequency domain symbols. The frequency domain symbols are input to an equalizer 125. The equalizer 125 eliminates the channel effect from the input frequency domain symbols and then outputs the symbols from which the channel effect has been eliminated. The symbols output from the equalizer 125 are input to a PSC 126. The PSC 126 converts the input parallel symbols into serial symbols and then outputs the serial symbols. The symbols serially converted in the unit of N symbols are input to a decoder 127. The decoder 127 decodes the input symbols and then provides an output bit stream 128.
The above-described OFDM system can more efficiently use a transmission band in comparison with a single carrier modulation system. For this reason, the OFDM system is widely used for a broadband transmission system.
In terms of reception characteristics, the OFDM system is more robust to a frequency selective multipath fading channel in comparison with a single carrier transmission system. Because there are a frequency selective channel in a frequency band occupied by multiple subcarriers and a frequency nonselective channel in each subcarrier band in terms of input signal characteristics of a receiver, a channel can be easily compensated in a simple channel equalization process. In particular, the OFDM system copies a second half part of each OFDM symbol, attaches the copied part as a CP before the OFDM symbol, and transmits the OFDM symbol, thereby removing ISI from a previous symbol. Thus, the OFDM transmission scheme is robust to the multipath fading channel and is proper for broadband high-speed communication.
In a standard for digital broadcasting, the OFDM transmission scheme receives attention as a transmission scheme capable of ensuring high quality of reception and high-speed transmission and reception. Examples of broadcasting standards adopting the OFDM transmission scheme are DAB for European wireless radio broadcasting, Digital Video Broadcasting-Terrestrial (DVB-T) serving as a terrestrial High Definition Television (HDTV) standard, and the like. Recently, a mobile broadcasting system is being developed in line with the global trend towards the convergence of broadcasting and communications. In particular, a major object of the mobile broadcasting system is to transmit a large amount of multimedia information. In Europe, DVB-Handheld (DVB-H) developed from DVB-T has been adopted as the mobile broadcasting standard. In South Korea, terrestrial Digital Multimedia Broadcasting (DMB) developed from DAB has been adopted as the broadcasting standard along with European DVB-H. MediaFLO proposed by Qualcomm is also based on the OFDM transmission scheme.
When a reception stage receives a signal modulated and transmitted by a transmission stage and converts the received signal into a baseband signal, synchronization between a transmission frequency and a reception frequency may not be acquired due to a tuner characteristic difference between the transmission stage and the reception stage. Herein, a frequency difference is referred to as a frequency offset.
Because this frequency offset leads to a decrease in signal magnitude and interference between adjacent channels, its correction is important to determine the performance of the OFDM system.
To correct the frequency offset in the OFDM scheme, many algorithms have been proposed. Synchronization algorithms for the OFDM system are divided into a carrier frequency synchronization algorithm and a symbol timing synchronization algorithm. The carrier frequency synchronization algorithm performs a function for correcting a carrier frequency offset between a transmitter and a receiver. The carrier frequency offset is caused by an oscillator frequency difference between the transmitter and the receiver, and a Doppler frequency offset. The carrier frequency offset of a signal input to a reception stage may be more than a subcarrier interval. A process for correcting an associated carrier frequency offset corresponding to an integer multiple of the subcarrier interval is defined as “initial carrier frequency synchronization.” A process for correcting an associated carrier frequency offset corresponding to a decimal multiple of the subcarrier interval is defined as “fine carrier frequency synchronization. A transmitted OFDM signal is shifted by an integer multiple of a subcarrier unit in a frequency domain due to an offset corresponding to an integer multiple of a subcarrier unit and therefore an FFT output sequence is shifted by the integer multiple of the subcarrier unit.
On the other hand, the carrier frequency offset corresponding to the decimal multiple of the subcarrier leads to interference between FFT outputs and significant degradation of Bit Error Rate (BER) performance. In general, it is known that the OFDM system has a larger amount of performance degradation due to the carrier frequency offset in comparison with the single carrier transmission system.
Existing initial carrier frequency synchronization algorithms for the OFDM system can be divided into a blind detection algorithm and an algorithm using a predefined symbol. In an example of the blind detection algorithm, a shift amount of a signal band is estimated using a guard band. However, it is difficult to actually implement the blind detection algorithm because performance degradation is very large under a multipath fading channel environment. On the other hand, the algorithm using the predefined symbol is disadvantageous in that a data transmission rate is reduced because the predefined symbol is transmitted independent of a data symbol. However, the algorithm using the predefined symbol is widely used for many OFDM systems because the performances of synchronization and channel estimation are improved.
In general, the predefined symbol transmitted for synchronization and channel estimation of the reception stage is constructed with a sequence capable of using autocorrelation characteristics like a Pseudo Noise (PN) sequence. FIG. 2 illustrates a sequence offset related to autocorrelation characteristics of a Phase Reference Symbol (PRS) serving as a predefined symbol used in a DAB system. When an offset of a conventional PN sequence is 0, a maximal autocorrelation value is provided. In the other case, the autocorrelation value is very small. However, in the case of the PRS of FIG. 2, it can be seen that a significantly large side peak occurs. In other offsets, a very small autocorrelation value appears.
As the initial carrier frequency synchronization algorithm using the predefined symbol, algorithms proposed by Nogami and Taura are well known. The algorithm proposed by Nogami is illustrated in FIGS. 3A and 3B.
First, a PN detector 320 detects an autocorrelation value of a PN sequence in a frequency domain during a predefined symbol. After the PN detector 320 detects the autocorrelation value of the PN sequence, a magnitude generator 330 performs a square operation on an absolute value and inputs a metric value Zn for a frequency offset to a maximal value-related index generator 350.
The metric value Zn is expressed as shown in Equation (1).
                                          Z            n                    ⁡                      (                          f              n                        )                          =                                                                        ∑                k                            ⁢                                                Y                  ⁡                                      [                    k                    ]                                                  ⁢                                                      p                    *                                    ⁡                                      [                                          k                      -                                              f                        n                                                              ]                                                                                            2                                    (        1        )            
Herein, Y[k] is a k-th FFT output result for an OFDM symbol in a PRS position, fn is an integer multiple of a carrier frequency offset estimate, p[k−fn] is a local PRS of a receiver with respect to fn, and p*[k−fn] is a complex conjugate of p[k−fn]. The maximal value-related index generator 350 estimates a frequency deviation value as an initial carrier frequency offset when an autocorrelation value is maximal.
Because the algorithm proposed by Nogami as illustrated in FIG. 3A is very sensitive to a symbol timing offset, an additional algorithm has been proposed which can reduce sensitivity to the symbol timing offset by decreasing an autocorrelation length and increasing a noncoherent combining length as illustrated in FIG. 3B.
Referring to FIG. 3B, a PN detector 320 detects an autocorrelation value of a PN sequence in a frequency domain during a predefined symbol. After the PN detector 320 detects the autocorrelation value of the PN sequence, a magnitude generator 330 performs a square operation on an absolute value. A second accumulator 340 accumulates an output of the magnitude generator 330 and inputs a metric value Zn to a maximal value-related index generator 350.
The metric value Zn is expressed as shown in Equation (2).
                                                        Z              n                        ⁡                          (                              f                n                            )                                =                                    ∑              m                        ⁢                                                                            R                  ⁡                                      [                    m                    ]                                                                              2                                      ⁢                                  ⁢                              R            ⁡                          [              m              ]                                =                                    ∑                              k                =                                  mN                  1                                                                                                  (                                          m                      +                      1                                        )                                    ⁢                                      N                    1                                                  -                1                                      ⁢                                          Y                ⁡                                  [                  k                  ]                                            ⁢                                                p                  *                                ⁡                                  [                                      k                    -                                          f                      n                                                        ]                                                                    ,                                  ⁢                  m          =          0                ,        1        ,        2        ,        …                            (        2        )            
Herein, Y[k] is a k-th FFT output result for an OFDM symbol in a PRS position, fn is an integer multiple of a carrier frequency offset estimate, p[k−fn] is a local PRS of a receiver with respect to fn, x* is a complex conjugate of x, and N1 is an accumulation length of a first accumulator. The maximal value-related index generator 350 estimates a frequency deviation value as an initial carrier frequency offset when an autocorrelation value is maximal.
On the other hand, the algorithm proposed by Taura corrects a PN sequence in a frequency domain, transforms the frequency domain sequence into a time domain sequence, and estimates a frequency shift amount mapped to a maximal value as an initial carrier frequency offset. This algorithm is significantly robust to a symbol timing offset, but requires very high hardware complexity because an IFFT process should be performed to compute every frequency offset estimate.
Among the conventional initial carrier frequency synchronization technologies in an OFDM receiver, the algorithm proposed by Nogami is difficult to be applied because autocorrelation characteristics are degraded when an FFT timing offset is large in a reception stage. That is, the FFT timing offset leads to linear phase rotation in the frequency domain. Thus, an autocorrelation length is reduced due to a limitation in the number of subcarriers capable of taking autocorrelation. As the autocorrelation length decreases, an autocorrelation value decreases and detection performance is degraded even though noncoherent combining is performed because distortion easily occurs due to a noise component. If an offset value is very large although FFT timing is detected, it can be seen that the performance of initial carrier frequency synchronization acquisition is significantly degraded in Nogami's algorithm.
On the other hand, when the FFT timing offset of the reception stage is small and interference from a previous symbol is absent under a multipath channel environment, only multipath components with a relatively small timing offset provide a large autocorrelation value and only multipath components with a relatively large timing offset provide a small autocorrelation value. In a Single Frequency Network (SFN) and a multipath channel environment with large channel delay spread, an amount of performance degradation further increases in Nogami's algorithm.
Among the conventional initial carrier frequency synchronization technologies in the OFDM receiver, the algorithm proposed by Taura can detect a predefined symbol even when an FFT timing offset is large, but has a disadvantage in that an IFFT process with very high hardware complexity should be used for processing in the time domain. In particular, the algorithm proposed by Taura is difficult to be used when a frequency offset is large because the IFFT process should be performed for one frequency estimate. Because only a multipath component with a largest magnitude value is used after transformation into the time domain, the number of multiple paths increases. There is a disadvantage in that performance is significantly degraded when the magnitudes of multipath components are similar to each other.
Accordingly, there is a need for an improved apparatus and method for carrier frequency synchronization in an OFDM system that sustains performance in the presence of multipath interference.